1. Field of the Invention
The present invention relates generally to a method and an apparatus for transmitting and receiving data, and more particularly, to transmitting and receiving data in a 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) system and an LTE Advance (LTE-A) system.
2. Description of the Related Art
In addition to providing a voice based initial service, a mobile communication system has evolved into a high speed and high quality wireless packet data communication system to provide a data service and a multi-media service. Various mobile communication standards such as High Speed Packet Access (HSPA) and LTE of 3GPP, High Rate Packet Data (HRPD), Ultra Mobile Broadband (UMB) of 3GPP2, and 802.16e of IEEE have been developed to support a high speed and high quality wireless packet data transmission service.
The newest mobile communication system uses an Adaptive Modulation and Coding (AMC) scheme and a channel response scheduling scheme to improve transmission efficiency. When using the AMC scheme, a transmitter may adjust an amount of data to be transmitted according to a channel state. That is, the newest mobile communication system may efficiently transmit a large amount of information in such a manner that the transmitter increases and reduces the amount of data to be transmitted to obtain a reception error probability of a desired level when a channel state is excellent and poor, respectively. When using the channel response scheduling scheme, because the transmitter selectively provides a service to a receiver having an excellent channel state among a plurality of receivers, system capacity is increased compared to when the transmitter provides a service after allotting a channel to one receiver.
For example, the AMC scheme and the channel response scheduling scheme receive feedback of partial Channel State Information (CSI) from a receiver, and apply suitable modulation and encoding techniques at the most efficient determining time point. The AMC scheme and the channel response scheduling schemes are techniques that may improve transmission efficiency in a state that a transmitter acquires sufficient information on a transmission channel. As in a Frequency Division Duplexing (FDD) scheme, when a transmitter does not recognize a state of a transmission channel through a receiving channel, a receiver is designed to report information about a transmission channel to the transmitter. Meanwhile, in the Time Division Duplexing (TDD) scheme, the receiver may not report information about a transmission channel to the transmitter using a characteristic capable of recognizing a state of the transmission channel through a receiving channel.
In recent years, research for changing a Code Division Multiple Access (CDMA) scheme being a multiple access scheme used in 2nd Generation Partnership Project (2G) and 2G mobile communication systems from a next generation system to Orthogonal Frequency Division Multiple Access (OFDMA) has been actively performed. Standard organizations such as 3GPP, 3GPP2, and IEEE are attempting standardization with respect to an evolution system using an OFDM or a modified OFDMA, because capacity may be increased in the OFDMA scheme in comparison with the CDMA scheme. One of various factors increasing capacity in the OFDMA scheme is to perform Frequency Domain Scheduling in a frequency axis. As a capacity gain may be obtained through a channel response scheduling scheme according to a channel changing according to time, a greater capacity gain may be obtained when a channel changes according to frequency.
An LTE system uses an OFDMA scheme in a Downlink (DK) and a Single Carrier Frequency Division Multiple Access (SC-FDMA) scheme in an Uplink (UL), and the two schemes involve performing scheduling in a frequency axis.
Meanwhile, a DL of the LTE system supports multiple antenna transmission. A transmitter of the LTE system may include one, two, or four transmission antennae. When the transmitter includes a plurality of transmission antennae, it applies pre-coding to obtain a beam forming gain and a spatial multiplexing gain. In the LTE system, a receiver transmits a Common Reference Signal (CRS) for each transmission antenna to estimate a channel response from each transmission antenna. When adaptive pre-coding is applied to data transmission, a transmitter reports to the receiver to which pre-coding is applied, through a control channel.
A DL of the LTE-A system increases the number of transmission antennae of a transmitter to support a maximum of eight transmission antennae. If the number of the transmission antennae increases, the transmitter may further improve the beam forming gain and the spatial multiplexing gain. However, so as to support the maximum eight transmission antennae, the amount of CRS for estimating a channel state from each transmission antenna is increased. In the LTE-A system, to prevent rapid increase in a rate of the CRS due to the increase in the number of transmission antennae, a receiver uses referring CSI-RS to create CSI feedback information and referring Demodulation-RS (DM-RS) for data demodulation. In the LTE-A system, a newly applied transmission scheme uses CSI-RS and DM-RS. However, to maintain backward compatibility, an LTE-A transmitter should also transmit CRS.
The LTE-A system supports Multimedia Broadcast Multicast Service Single Frequency Network (MBSFN) transmission, which spreads a service zone of broadcasting by transmitting the same broadcasting signal from multiple cells. The MBSFN transmission regulates to transmit the same RS from multiple cells such that a receiver may perform coherence modulation for the simultaneously transmitted signal without cell separation. That is, because the MBSFN cannot be supported with the CRS defined by cells, only an MBSFN dedicated RS is defined in an MBSFN sub-frame and is transmitted. Further, because a unicast service is not supported in the MBSFN sub-frame, the CRS is not transmitted.
The LTE system regulates a plurality of transmission modes to select a suitable transmission scheme according to a state of a receiver, and uses a CRS and a transmission scheme using a Dedicated Reference Signal (DRS) for channel estimation for demodulation. The LTE-A system adds a transmission scheme using DM-RS for channel estimation for demodulation. A receiver of the LTE-A system should be able to receive all of CRS, DRS, and DM-RS in consideration of backward compatibility.
The receiver of the LTE-A system may also receive a unicast service from an MBSFN sub-frame. The DM-RS may be transmitted to an MBSFN sub-frame instead of the CRS because the MBSFN sub-frame is regarded as a source without backward compatibility. Herein, in order to distinguish from an MBSFN sub-frame set for supporting an MBSFN service, a sub-frame set for supporting a unicast service to an MBSFN sub-frame is referred to as an LTE-A sub-frame.
In the foregoing LTE-A system, an operation of a receiver for receiving a unicast service in the MBSFN sub-frame should be defined. When a receiver of the LTE-A system is set to a transmission mode using a CRS for demodulation, it may receive data in a general sub-frame. However, since the CRS is transmitted in the MBSFN sub-frame, although the receiver of the LTE-A system may perform reception using DM-RS, the receiver cannot receive data.